Sound field reproduction of concert halls on the Internet

Work plan in 2004

Aim of research

We aim to develop a technique to realize the sound field reproduction of
concert halls at low cost. It is possible to reproduce the concert hall sound
field by converting a monaural source to 2ch or 4ch signals using the
information of reflective sound (direction, amplitude, and time delay)
calculated by the acoustic CAD software. By this approach, we do not need a
large-scale multi-speaker system as before. Also, the use of the acoustic CAD
software allows to reduce cost of acoustic measurement in the actual halls. We
expect that this project will contribute to the realization of the seat
selection system of concert halls on the Internet.

Generally, the criterion of selecting a seat in a concert hall is a good
stage view. The ticket price becomes expensive for the seats close to the stage.
It is not widely recognized, however, that the sound quality changes a lot by
the position inside the hall. We have performed the acoustic measurement in two
Japanese concert halls and found that the acoustic qualities, such as the sound
level, the reverberance, and the spaciousness, are quite different between seat
positions in the same hall. We think that the audiences can obtain higher
satisfaction if they know their preference of sound and listen to the music at
their suitable seats.

In order to enable listeners to get to know their sound preference, it is
required to give them chance of experiencing the sound in the halls. An example
of equipment for this purpose is the sound simulation room
in the Kirishima International Music Hall. The 16ch surround sound system is
installed in this room. However, installing such equipment in every hall is
actually difficult. There is another restriction that people cannot experience
if they do not visit that place. Therefore, we propose a sound field
reproduction system available on the Internet.

The technique for the concert hall sound field reproduction is called
Auralization, and many different methods have been proposed. Among them, one of
the most widely used technique is the multi-channel reproduction system. In this
system, the loudspeakers are placed at various direction of the listener so that
the reflective sounds are reproduced from them depending on their direction.
This system provides a good sound localization without any special signal
processing, but needs many channels for minimizing the localization error. For
our purpose, the auralization system with less channels is required.

Reproduced signal in the multi-channel system is a convolution of the music
signal (dry source) and the impulse response in the concert hall. The impulse
response contains information of reflective sound (its direction, strength, and
time delay) arriving after the direct sound. To reproduce the sound fields at
every seat positions in the hall, we need many impulse responses recorded at
those positions. However, it takes much time and cost. Therefore, it is
reasonable to use the calculated impulse response by use of the acoustic
simulation.

Work plan

Calculation of the impulse response by the acoustic CAD software

In the acoustic CAD software, we can calculate the direction, the amplitude,
and the time delay of reflections at any particular position inside the hall, by
using the image source method. An example of the calculation result is shown
below. In this figure, the path of the reflective sounds are represented as
colored lines. Different colors indicate the different order of reflection. In
this example, first to third order reflection is shown.

Figure 1: an example of the simulation result by the image source method

In the computation of the reflection path, it takes much time for selecting
the reflective surface and for detecting the interrupting surface on the
reflection path. In our original acoustic CAD software, YMCAD, we reduce the
computation time by narrowing down the candidates of reflecting surface and the
interrupting surface beforehand. As compared with the conventional method, our
method improves the computation speed of about 1000 times when up to fifth
reflections are considered.

In the image source method the impulse response is also calculated. Each
reflection contains information of its amplitude, direction, and time delay from
the direct sound. The multi-channel reproduction system decomposes this impulse
response according to the direction of a speaker, then outputs each reflection
with appropriate time delay. Separately reproduced reflections are then summed
up in the listening room, and the original impulse response is reconstructed .

Figure 2: an example of the impulse response calculated by the image source
method

Auralization based on the 5.1 surround sound system

5.1 surround sound system consists of six loudspeakers: center front (C),
left and right front (L, R), left and right surround (LS, RS), and subwoofer
(this is counted as 0.1ch). A typical layout of the loudspeakers is shown below.
In this configuration, sound localization in left-right and front-rear becomes
possible.

Figure 3: a layout of the loudspeakers in 5.1 surround sound system(1)

In our proposed system, the impulse response calculated by the image source
method is reconstructed by five speakers (excluding 0.1 channel subwoofer,
because it does not affect localization). First, the direct sound is fed into
the center front speaker (C). If we assume that a listener is always faced to
the musician on the stage, the direct sound always come from listener's front.
Next, the reflective sounds coming from the forward of the listener are
reproduced by center two speakers (L and R). By introducing the appropriate time
difference and level difference between two speaker outputs, direction of each
reflective sound will be perceived correctly by the listener. Finally, the
reflections coming from the backward are fed into LS and RS similarly.

Figure 4: block diagram of the proposed system

Based on the methods described above, we are now developing the software to
create a 5.1ch sound file from a monaural dry source. We expect that this
enables the distribution of the simulated sound field of concert halls by using
simple speaker configurations.